The present invention relates to a zinc oxide-based nonlinear voltage dependent resistor for lightning arrestors and to a method for manufacturing thereof, and more particularly relates to a nonlinear voltage dependent resistor with a high impulse current withstand property and a method for manufacturing thereof.
A zinc oxide-based nonlinear voltage dependent resistor is produced through a well-known ceramic sintering technique. Starting materials including zinc oxide (ZnO) powder as the main component, bismuth oxide (Bi.sub.2 O.sub.3), antimony oxide (Sb.sub.2 O.sub.3), cobalt oxide (Co.sub.2 O.sub.3), manganese oxide (MnO.sub.2), chromium oxide (Cr.sub.2 O.sub.3), silicon oxide (SiO.sub.2), boron oxide (B.sub.2 O.sub.3), and aluminum oxide (Al.sub.2 O.sub.3) are well mixed with each other. After adding a suitable binder such as water or polyvinyl alcohol to the mixture, the resulting mixture is granulated, and the granules are molded. The obtained molding is fired or sintered at high temperatures. In order to prevent flashover, an inorganic paste comprising a mixture of a SiO.sub.2 -Sb.sub.2 O.sub.3 -Bi.sub.2 O.sub.3 ternary component and an organic binder is coated to the sides of the sintered body, dried and baked in an electric furnace at a temperature of 800 to 1,500.degree. C., thus high resistance side layer is formed around the sintered body, as disclosed for example in Japanese Pat. Publication No. 53-21516 published on Jul. 3, 1978. Each of the upper and lower ends of the nonlinear voltage dependent resistor thus produced is ground to obtain a desired thickness and electrodes are formed on these ends by metal spraying or baking to form a product. In order to increase the impulse current withstand property, or in other words flashover withstand ability, of the nonlinear voltage dependent resistor, the thickness of the high resistance side layers has to be increased, however, which causes interfacial cracking or peeling of the high-resistance side layers from the nonlinear voltage dependent resistor body during the baking process due to the difference of thermal expansion coefficients between the body and the high-resistance side layers, so that a flashover is apt to occur even at a relatively low impulse current applied.
A method for forming a high-resistance side layer by diffusing lithium or its compound is also known as disclosed for example in Japanese Pat. Publication No. 5221714 published on Jun. 13, 1977. However, this method has drawbacks that a control of the thickness of the high-resistance side layer is difficult, since lithium ions are diffused among zinc oxide crystal grains and that the lithium ions are diffused into the inside of the element, the nonlinear voltage dependent resistor body, to damage its nonlinearity when the element is used for a long period of time.